This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-195051, filed Jun. 27, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates generally to an antenna apparatus, and more particularly to an antenna apparatus suitable for an MSB (Mobile Satellite Broadcast) system.
2. Description of the Related Art
With an increase in the demand for communications, remarkable developments have been achieved in these years in the field of communication and broadcasting. In the near future, a mobile broadcasting system, which makes use of artificial satellites (hereinafter referred to as xe2x80x9csatellitesxe2x80x9d) such as broadcasting satellites and communications satellites, will be put to practical use.
In the mobile broadcasting system, it is important that radio waves from satellites can always be received by mobile stations in good condition. In big cities, however, high-rise buildings, etc. are obstacles to radio communication, and mobile stations are unable to directly receive radio waves from the satellites in most cases. To solve this problem, there is an idea that relay stations are disposed at locations open to the satellites without obstacles. Radio waves from the satellites are re-broadcast to ground areas via the relay stations. Thereby, the condition of radio reception by the mobile stations is always kept constant.
In addition, in this system, it is necessary to receive both radio waves from satellites and radio waves from relay stations with desired gains. Radio waves come from the satellites at a certain elevation angle, while radio waves come from the relay stations in the substantially horizontal direction. These matters, in particular, have to be considered in designing reception antennas.
In Japan, for example, a satellite is situated to have an angular range of 35xc2x0 to 60xc2x0 with respect to the zenith (0xc2x0). Thus, a reception antenna is required to have a performance capable of receiving radio waves from directions at these angles with a gain of about 2.5 dBi or more. In addition, the reception antenna is required to have a performance capable of receiving radio waves coming from the relay stations in a substantially horizontal direction with a gain of about 0 dBi.
Patch antennas designed for a GPS (Global Positioning System), etc. may possibly be applied to the antennas for the above system. However, the conventional antenna apparatus is specifically designed for receiving radio waves coming from the sky. The conventional antenna apparatus has a disadvantage that the reception gain in the horizontal direction is low.
To solve this problem, there is an idea that an additional antenna for receiving radio waves in the horizontal direction is provided. According to this solution, however, the cost rises, the size and weight of the antenna apparatus increase, and the external appearance is disadvantageously degraded. In particular, these drawbacks have to be eliminated in the case where the antenna apparatus is mounted on vehicles, etc.
As has been described above, the conventional antenna apparatus is specifically designed for receiving radio waves from satellites, and the gain of horizontal reception in this antenna apparatus is low. Thus, this antenna apparatus cannot suitably be applied to the next-generation mobile communication systems. Moreover, since the conventional antenna apparatus requires a purpose-specific antenna for obtaining a horizontal reception gain, the size, weight and cost will disadvantageously increase.
The object of the present invention is to provide an antenna apparatus at low cost, which is capable of obtaining a reception gain in a desired range of elevation angles, without increasing the size and weight of the apparatus.
In order to achieve the above object, this invention may provide an antenna apparatus comprising: a base plate having one surface provided with a ground conductor; a helical antenna section attached to the base plate and provided with an n-number of antenna elements helically formed at an angular distance of (360/n)xc2x0 therebetween; and a radio frequency signal supplier which is provided on the other surface of the base plate and supplies power to the n-number of antenna elements of the helical antenna with a phase difference of (360/n)xc2x0.
In this antenna apparatus, it is preferable that the helical antenna section may comprise a four-wire helical antenna having four antenna elements arranged at an angular distance of 90xc2x0 therebetween, and that the radio frequency signal supplier supply power to the four antenna elements of the helical antenna section with a phase difference of 90xc2x0.
It is preferable that when a reception wavelength is xcex, the height of the helical antenna section be 0.6 xcex to 0.75 xcex. It is also preferable that the number of turns of each antenna element be about 1. It is also preferable that each antenna element of the helical antenna section have a pitch angle of about 50xc2x0 to 60xc2x0.
In this invention, each of the antenna elements may include a straight portion extending parallel to the axis of the helical antenna.
Preferably, the straight portion is formed with a length corresponding to about xc2xc of the height of the helical antenna section at a location corresponding to xc2xd to xc2xe of the height of the helical antenna section from the ground conductor. Preferably, when a reception wavelength is xcex, the height of the helical antenna section is about 0.3 xcex to 0.35 xcex. More preferably, the height of the helical antenna section is about 0.34 xcex. Preferably, the number of turns of each antenna element is about 1. Preferably, each antenna element of the helical antenna section has a pitch angle of about 22xc2x0.
Moreover, in this invention, the helical antenna section may be attached to a surface of the base plate, on which the radio frequency signal supplier is provided.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.